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Efield integral

  1. Oct 1, 2013 #1
    this is the E field above a square loop with side=a at distance z on the z axis. by symmetry the Ex, Ey field cancel out. it really kind of bothers me that I can't see this substitution, including the sin(tan^-1(u)) portion. I guess it's an easy technique but I wonder how you get the substitution?

    [tex]Ez=\frac{4\lambda z}{4\pi \varepsilon o}\int_{-a/2}^{a/2}\frac{ dx}{(z^2+x^2+a^2/4)^{3/2}}, x=\sqrt{a^2/4+z^2}tanu, dx=\sqrt{a^2/4+z^2}sec^{2}udu, I=\frac{1}{a^2/4+z^2}\int cosudu=\frac{1}{a^2/4+z^2}sinu\therefore Ez=\frac{8\lambda az}{4\pi\epsilon o \sqrt{2a^2+4z^2}z^2+a^2/4}[/tex]


    thanks!
     
  2. jcsd
  3. Oct 1, 2013 #2

    arildno

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    Thumb rule:
    "To remove squares within square roots, use a suitable trig substitution. If stuff is more difficult than that, forget about it"
     
  4. Oct 1, 2013 #3
    I try to do these by hand- I wonder if some people do all of these by hand?
     
    Last edited: Oct 1, 2013
  5. Oct 1, 2013 #4

    arildno

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    Those who need to learn them, such as students. Those who are professionally competent don't, but at most, picks up a table of integrals. Only those are professionally competent who has learned them.

    :smile:
     
  6. Oct 2, 2013 #5
    I'm not sure if maybe it's better to do a lot of problems out of mathematical and Schaum books. I had another career before this that was completely different than physics/mathematics
    I've been looking at Apostol and Hubbard Calculus (introduces manifolds) and I'm not sure if I should study these since I've already done Stewart (problems are easy I know)
     
  7. Oct 2, 2013 #6

    arildno

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    I'm not sure what you're getting at.

    Ask yourself:
    If lots of the basic tools used within research seems utterly magical to you, because you haven't learnt the logic behind them, can you ever succeed as a researcher? There's a reason why an education within physics/math is a fast-track over the evolution of the subjects, ordered in a pedagogically optimal way.
    Alternatively: Why should anyone want to employ you, unless you have a proven ability to understand and master previous areas of research?

    -------------------------------
    Thus, the function behind drilling students with such as nasty integrals is at least three-fold:
    To hone the mind of the student, and familiarize him with the technical language, along with being a quality control of the student.
     
  8. Oct 3, 2013 #7
    thanks, it's not a difficult substitution.

    so the integral substitution works well with the tan^2+1 term reducing it to cosu and you just substitute back in for u and solve,
    [tex]sin(arctan(\frac{x}{(a^2/4+z^2)^{1/2}}))=\frac{x}{(a^2/4+x^2+z^2)^{1/2}}[/tex]
     
    Last edited: Oct 3, 2013
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